1. Field of Invention
The invention relates to a semiconductor device and the associated manufacturing process. In particular, it relates to a manufacturing method of the selection gate in a split-gate flash EEPROM cell.
2. Related Art
Typically, the data storage media in computers can be separated into volatile and nonvolatile memory. The volatile memory includes the dynamic random access memory (DRAM) and static random access memory (SDRAM). Since the data stored in such memory will disappear immediately after the power supply is interrupted, it is mainly used in temporary data input/output (I/O). The nonvolatile memory can keep the stored data even after the power supply is turned off. Therefore, such memory can be used in various occasions. The nonvolatile memory can be divided according to the access method into mask read only memory (ROM), erasable programmable read only memory (EPROM), electric erasable programmable read only memory (EEPROM), and flash EEPROM.
Since the introduction of the 256K flash EEPROM in 1987, it has gradually become the mainstream of nonvolatile memory. The flash EEPROM is a type of high-density memory that combines the advantages of EPROM and EEPROM. It has the merits of being nonvolatile, rewriteable, high-density, and long-lasting. Therefore, it is ideal for applications in portable computers and telecommunications. Some scholars even predict that the flash EEPROM will start the next semiconductor evolution. We thus see the importance of the flash EEPROM in the semiconductor industry.
Normally, the flash EEPROM can be divided according to the structure into split-gate and stack-gate ones. The data erasing speed of the split-gate flash EEPROM is faster than that of the stack-gate ones. Therefore, the semiconductor industry favors the former. In general, the structure of the split-gate flash EEPROM cell includes: a suspending gate consisted of a gate oxide/polysilicon/oxide structure, a control gate/selection gate, and an inter polysilicon dielectric layer between the suspending gate and the control gate/selection gate. It is called the suspending gate structure because its polysilicon layer does not have any electrical communications with any electrode. The data erasing and writing actions in the flash EEPROM can be achieved by imposing different voltage combinations on the gate, source, drain, and substrate, injecting or moving out electrons from the suspending gate. In order for the split-gate flash EEPROM to operate correctly, the selection ate has to at least cover the distance between the drain (or source) and the suspending gate. That is, an appropriate distance has to be maintained between the drain or source and the suspending gate as the electron channel. If the channel length is too short, short channel effects may happen; while if the channel length is too long, the writing efficiency will be bad.
With the increase in the semiconductor integration, the device sizes have been shrunk down to submicron or deep submicron (<0.35 μm) scales. However, normal operations of the flash EEPROM require an appropriate channel length. Therefore, there is some difficulty in making submicron-scale split-gate flash EEPROM. A new split-gate flash EEPROM structure that allows for a certain channel length in the submicron scales is required.